1. Field of the Invention
The object of this invention is a method for dosing small amounts of liquid in a quantitative way by using a liquid space, the volume of which is changeable and which is connected to a dosing channel so that suction or spraying is achieved through the dosing channel by increasing or decreasing the volume of the liquid space.
2. Discussion of the Related Art
In wet-chemical analyzers used for clinical purposes or the like, the dosing of liquid is typically carried out by measuring a few micro liters of a sample and a few tens of micro liters of a reagent and by adding a diluting agent so that the total volume of the dosed item of liquid is about 100-1000 .mu.l. A liquid space, the volume of which is changeable, is used in the dosing, the liquid space being formed by a ground glass cylinder provided with a plunger sealed with teflon, and which communicates with the dosing tip through the channel, the liquid being able to be injected from the tip to a vessel such as a small rinsing basin.
The movement of the plunger in the cylinder, on which the dosing of the liquid is based, is generally provided by a stepper motor which rotates, through a reduction gear and through a cogged belt, and a slot-headed screw. The piston rod is suspended on the nut on the slot-headed screw through a transversal arm. The maximum length of the movement of the plunger can be about 60 mm which requires 3000-6000 steps of the stepper motor. The precision of the dosage with such a number of steps can be in the order of 0.5-1% from the total volume of the cylinder and the repeatability 0.02-0.05% from the total volume, i.e., about 1-3 steps of the stepper motor. However, if only 1% of the volume of the cylinder is used for dosing, the repeatability of the dosage is in the order of about 1.5% from said volume.
The above values indicate that the present dosing techniques cannot provide very good dosing precision and that the precision and the repeatability further depend very much on the magnitude of the dosage relative to the total volume of the cylinder. One of the reasons for this is the fluctuation of the friction between the plunger and the cylinder which leads to inaccurate starting-up and stopping of the plunger. Another considerable reason for the inaccuracy is the multi-phase transmission chain between the stepper motor and the plunger, the sum of the tolerances of the transmission chain being high and the repeatability of movements low because of this. In addition, the mutual differences of the steps of the stepper motor, which are nominally of the same length, are actually considerably large. This problem has been perceived in present liquid dosing members and the intention has been to compensate for it by using a large number of steps so that the mutual differences of steps are equalized. However, the result is that 1/20-1/50 of the total volume of the cylinder is typically the smallest amount of liquid which can be dosed with a reasonable repeatability of 1%. When smaller dosages are needed, a dosing space with a smaller cross-section is required. Because of this it has been necessary to provide the present analyzers with numerous different dosing members which increase the size and the price of the analyzer.
One of the main problems of the present dosing techniques is the wearing of the walls of the cylinder and that of the teflon-seal of the plunger which moves in the cylinder. This further decreases the precision of the dosing and, therefore, the condition of the parts should be monitored. The monitoring and changing of the parts, in turn, increase the operating expenses of the equipment. In order to mitigate the problem of wearing, the teflon-seals have been made very massive but then the friction increases so that it has been necessary to also make the reduction gear transmitting the movement of the plunger more massive. Thus it was necessary to place the dosing device, because of its size, separately from the dosing point and the cylinder is joined to the moving dosing tip by a plastic tube. The movements, deformation, increase of the area and the volume, and thermal expansion of such a tube further impair the repeatability of the dosing. Another alternative for decreasing the wearing of the parts is to slow down the movement of the plunger but this decreases the capacity of the dosing device which is a considerable drawback in an expensive analyzer which is required to be effective.